Any water in a sea or lake that is neither close to the bottom nor near the shore can be said to be in the pelagic zone. The word "pelagic" is derived the from Greek πέλαγος (pélagos), meaning "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes from the surface of the sea almost to the bottom. Conditions differ deeper in the water column such that as pressure increases with depth, the temperature drops and less light penetrates. Depending on the depth, the water column, rather like the Earth's atmosphere, may be divided into different layers.

The pelagic zone occupies 1,330 million km3 (320 million mi3) with a mean depth of 3.68 km (2.29 mi) and maximum depth of 11 km (6.8 mi).[1][2][3] Fish that live in the pelagic zone are called pelagic fish. Pelagic life decreases with increasing depth. It is affected by light intensity, pressure, temperature, salinity, the supply of dissolved oxygen and nutrients, and the submarine topography, which is called bathymetry. In deep water, the pelagic zone is sometimes called the open-ocean zone and can be contrasted with water that is near the coast or on the continental shelf. In other contexts, coastal water not near the bottom is still said to be in the pelagic zone.

The pelagic zone can be contrasted with the clams and crabs, are called benthos. The demersal zone is just above the benthic zone. It can be significantly affected by the seabed and the life that lives there. Fish that live in the demersal zone are called demersal fish, which can be divided into benthic fish, which are denser than water so they can rest on the bottom, and benthopelagic fish, which swim in the water column just above the bottom. Demersal fish are also known as bottom feeders and groundfish.

Contents

Depth and layers1

Epipelagic (sunlight)1.1

Mesopelagic (twilight)1.2

Bathypelagic (midnight)1.3

Abyssopelagic (lower midnight)1.4

Hadopelagic1.5

Pelagic ecosystem2

Pelagic birds2.1

Pelagic fish2.2

Pelagic invertebrates2.3

Pelagic reptiles2.4

References3

Further reading4

Depth and layers

A scale diagram of the layers of the pelagic zone

Depending on how deep the sea is, the pelagic zone can extend over up to five horizontal layers in the ocean. From the top down, these are:

Mesopelagic (twilight)

From 200 m (660 ft) down to around 1,000 m (3,300 ft)

The name for this zone stems from Greek μέσον (meson), meaning "middle". Although some light penetrates this second layer it is insufficient for photosynthesis. At about 500 m the water also becomes depleted of oxygen. Organisms survive this environment by having more efficient gills or by minimizing movement.

Examples of animals that live here are bioluminescent.[4] Some creatures living in the mesopelagic zone rise to the epipelagic zone at night to feed.[4]

Bathypelagic (midnight)

From 1,000 m (3,300 ft) down to around 4,000 m (13,000 ft)

The name stems from lanternfish. No living plant life exists here. Most animals living here survive by consuming the detritus falling from the zones above, which is known as "marine snow", or, like the marine hatchetfish, by preying on other inhabitants of this zone. Other examples of this zone's inhabitants are giant squid, smaller squids and the dumbo octopus. The giant squid is hunted here by deep-diving sperm whales.

Abyssopelagic (lower midnight)

From around 4,000 m (13,000 ft) down to above the ocean floor

The name is derived from Greek ἄβυσσος (ábyssos), meaning "bottomless" (a holdover from the times when the deep ocean, or abyss, was believed to be bottomless). Very few creatures live in the cold temperatures, high pressures and complete darkness of this depth.[4] Among the species found in this zone are several species of squid; echinoderms including the basket star, swimming cucumber, and the sea pig; and marine arthropods including the sea spider.[4] Many of the species living at these depths are transparent and eyeless because of the total lack of light in this zone.[4]

Hadopelagic

The deep water in ocean trenches

The name is derived from the realm of hydrothermal vents in this and other zones. Some define the hadopelagic as waters below 6,000 m (20,000 ft), whether in a trench or not. The bathypelagic, abyssopelagic, and hadopelagic zones are very similar in character, and some marine biologists combine them into a single zone or consider the latter two to be the same. The abyssal plain is covered with soft sludge composed of dead organisms from above.

Pelagic ecosystem

The pelagic sooty tern spends months at a time flying at sea, returning to land only for breeding.[5]

The pelagic ecosystem is based on the phytoplankton which occupy the start of the foodchain. Phytoplankton manufacture their own food using a process of photosynthesis. Because they need sunlight, they inhabit the upper, sunlit epipelagic zone, which includes the coastal or neritic zone. Biodiversity diminishes markedly in the deeper zones below the epipelagic zone as dissolved oxygen diminishes, water pressure increases, temperatures become colder, food sources become scarce, and light diminishes and finally disappears.[6]

The term seabird includes birds which live around the sea adjacent to land, as well as pelagic birds.

Pelagic fish

Pelagic fish live in the water column of coastal, ocean, and lake waters, but not on or near the bottom of the sea or the lake. They can be contrasted with demersal fish, which live on or near the bottom, and reef fish, which are associated with coral reefs.[7]

Pelagic invertebrates

Pelagic reptiles

Pelamis platura, the pelagic sea snake, is the only one of the 65 species of marine snakes to spend its entire life in the pelagic zone. It bears live young at sea and is helpless on land. The species sometimes forms aggregations of thousands along slicks in surface waters. The pelagic sea snake is the world’s most widely distributed snake species.

Many species of sea turtles spend the first years of their lives in the pelagic zone, moving closer to shore as they reach maturity.

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